Various industrial processes require the introduction of precise amounts of liquid or vaporized liquid. For many processes, the liquid or vapor delivery rate must be precisely measured and controlled to achieve acceptable results. Such processes include, for example, blood diagnostics, titration, dosing, chamber humidification, vapor deposition, stripping, annealing, and chemical etching.
Conventional liquid and vapor mass delivery systems rely on techniques such as displacement metering pumps, time based pressure dispense, thermal based flow controllers, ultrasonic, differential pressure, and Coriolis. Although each such system has specific shortcomings, a common disadvantage is their lack of immunity to entrained gases. To reduce the error caused by entrained gases many liquid flow controller manufacturers recommend the installation of degassing equipment. This equipment adds to the cost and complexity of the delivery system and also introduces another potential error, namely the efficiency of the degassing process.
Conventional vapor delivery systems include bubblers, evaporators, and flash vaporizers. With each such vapor delivery system multiple devices are required to deliver the vapors of the precursor to the process chamber. A disadvantage of bubblers is the resulting shift in the ratio of chemical vapor to carrier gas when the liquid temperature or bubbler pressure change. Although bubbler vapor output feed-back compensation techniques have been developed to compensate for these influences, they significantly increase the cost of the system. A disadvantage of evaporators is that a mass flow control device is still required to control and report the flow rate of the vaporized precursor. Furthermore the delivery of reactive chemical vapors can affect the accuracy of the flow control device or cause it to prematurely fail. A disadvantage of flash evaporators is that they require a liquid flow control device, such as a volumetric displacement pump or liquid mass flow controller to inject a precise amount of liquid into a heated chamber held at a temperature sufficient to vaporize the liquid. Such liquid injection techniques introduce potentially significant errors, and the temperature of the vaporizer must often be held at or above the decomposition temperature of the chemical.